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Re: Proposal for simplifying FILTER semantics

From: Markus Krötzsch <markus.kroetzsch@comlab.ox.ac.uk>
Date: Wed, 18 May 2011 11:19:45 +0100
Message-ID: <4DD39D41.90602@comlab.ox.ac.uk>
To: Axel Polleres <axel.polleres@deri.org>
CC: public-rdf-dawg-comments@w3.org
Axel,

thank you for your reply. As indicated in my post, there already is 
another way of faithfully expressing FILTERs in disjunctions in SPARQL 
1.0. So there is no need for increasing expressivity, only for syntactic 
simplification.

As far as I understand, BIND achieves this simplification, but only in 
the case where a FILTER uses "=". So most FILTER conditions can not be 
expressed with this method. A special encoding for binding "=" FILTERs 
hardly makes using SPARQL easier or more uniform in general, but I 
acknowledge that it addresses my concerns for a particularly important 
use case. Thus I fully support the introduction of BIND in SPARQL 1.1.

Notwithstanding any of the above remarks, I consider the working group's 
reply to be absolutely adequate, and I welcome the advance of SPARQL 1.1 
to Last Call.

Regards,

Markus


On 10/05/11 15:16, Axel Polleres wrote:
> Markus,
>
> This is in respons of your comments
>   http://lists.w3.org/Archives/Public/public-rdf-dawg-comments/2011May/0000.html
> and
>   http://lists.w3.org/Archives/Public/public-rdf-dawg-comments/2011May/0003.html
>
>
> I was looking at your page http://korrekt.org/page/The_State_of_the_UNION
>
> First of all, it seems you disregard the power of BIND and EXISTS, new constructs in SPARQL1.1 which seems to solve your use cases.
>
> E.g., BIND does the job for your first example:
>
>   SELECT ?x WHERE {
>    { ?x :population ?number FILTER( ?number>1000000 ) }
>    UNION
>    { BIND (:Luxemburg AS ?x ) ?x :population  ?number }
>   }
>
> and seems to cover your stated requirement that
>
> "* Define FILTER in such a way that it can *create* new solution mappings, just like BGP."
>
> The difference between FILTER and BIND - by design - is that BIND creates bindings while FILTER only filters solutions.
>
>
> As for your second requirement:
> "* Instead of applying filters after matching, the generated solution
> mappings of a FILTER would directly be joined with other parts of the
> query."
> let's turn to your second example
>
>   "Find all ?x locatedIn ?y, where ?y is the USA or a state of the USA."
>
> Here, similar to above
>
>   SELECT ?x WHERE {
>    { BIND (:USA AS ?y) ?x locatedIn ?y }
>    UNION
>    { ?y :stateOf :USA . ?x locatedIn ?y }
>   }
>
> seems to do the job. Similarly, a FILTERs only version is possible, however, within FILTERs you can't use UNION, but you can use || in combination with EXISTS here to express the desired filter conditions over patterns:
>
>   SELECT ?x WHERE {
>     ?x locatedIn ?y
>     FILTER  ( ?y = :USA || EXISTS( ?y :stateOf :USA ) )
>   }
>
> the additional condition
>
>   "?y has at least 10 million inhabitants or is in the pacific"
>
> can be added likewise.
>
> Summarising, I think your request
>
>   "Concretely, one would need to make the following modifications:
>      change the formal semantics to treat FILTERs like patterns, not
>      like post-processing features,
>   "
> is already addressed by the EXISTS feature of SPARQL 1.1 which readily provides this feature, and seems to address your use
> case. Additionally, the new BIND feature allows you to create bindings.
>
> Finally, as for your proposal in you second mail
>   http://lists.w3.org/Archives/Public/public-rdf-dawg-comments/2011May/0003.html
> Please note that your suggested change of the semantics would affect backwards compatibility with SPARQL1.0, which our charter explicitly discourages [1].
>
> We'd appreciate if you could indicate whether this response adequately addresses your comment.
>
> Regards,
>
> Axel (on behalf of the SPARQL WG)
>
> 1. http://www.w3.org/2009/05/sparql-phase-II-charter.html#scope
>
>


-- 
Dr. Markus Krötzsch
Oxford  University  Computing  Laboratory
Room 306, Parks Road, Oxford, OX1 3QD, UK
+44 (0)1865 283529    http://korrekt.org/
Received on Wednesday, 18 May 2011 10:20:10 GMT

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